Precision wire-wound resistor



nitd States Patent fiice 3,012,216 PRECISION WIRE-WOUND RESISTOR Kurt Coper, Brookhaven, Pa., assignor to The Daven Company, Livingston, NJ., a corporation of New Jersey Filed Mar. 16, 1959, Ser. No. 7 99,641 8 Claims. (Cl. 338322) The present invention relates to wire wound resistors and particularly to precision resistors of high ohmic value which may be used at elevated temperatures, up to about 200 C. for example, and wherein provision must be made to prevent ingress of moisture and humidity into contact with the resistive winding.

It is possible to encase a wire wound resistor in a moisture-resistant and practically moisture-tight shell of plastic material such, for example, as silicone rubber which has now become a well-known and quite adaptable type of plastic material capable of withstanding elevated temperatures of the order of magnitude of those abovereferred to without substantial loss of its moisture-resistant characteristics. It is found, however, that while the plastic material itself is substantially waterproof it does not provide a moisture-proof bond with the flexible lead-out conductors with which the resistor is necessarily provided so moisture may penetrate at these points and eventually find its way into contact with the insulated resistance wire. Air often contains small proportions of one or more gaseous salts or acids which under relatively moist conditions may be corrosive to metals or, where electrical potential diflerences prevail, may erode metals by electrolysis. The latter condition, inherently involving electrical conductivity, also tends substantially to impair the precision often desired in resistors of high ohmic value. Inasmuch as the insulated resistance wire used in wirewound resistors of high ohmic value is very fine (with a conductor diameter often as small as 0.001 inch), and the resistor in normal use has an electrical potential impressed across its terminals so that the conditions leading to electrolysis are present, the preservation of the desired precision and the protection of this delicate wire demand that the resistor be etiectively sealed against the entrance of moisture with its conductive and corrosive and erosive eflects.

The present invention provides a novel resistor having a particular configuration of the resistor core structure such that a compression type of moisture-proof seal not only closes oil? the resistor winding from the regions of the lead-in conductors, but greatly extends the length of the path over which moisture must travel by capillary action past the seals from its outside contact with the lead wires into contact with the resistance wire. Each lead wire itself extends through, and is mechanically anchored by, the core structure but is hermetically sealed at its point of entrance to the core, thus wholly preventing ingress of moisture into contact with the resistance wire except past the compression end seals and by way of the long and sinuous path between the core and the encasing plastic material shell as aforesaid.

The present invention, therefore, ensures that the delicate resistance wire of a wire-wound resistor is maintained sealed against the entrance of atmospheric moisture and humidity even under elevated operating temperature conditions and thus enables operation under temperature and humidity conditions far more severe than those which are tolerable for any appreciable length of time by conventional prior art resistors.

Other and more retailed objects and advantages of the present invention will become apparent from the following description of preferred forms of the invention, illustrated in the accompanying drawings, in which:

3,012,216 Patented Dec. 5, 1961 FIG. 1 is a View of the completed wire-wound resistor with its lead wires extending substantially axially in opposite directions;

FIG. 2 is an elevational enlarged-scale cross-sectional view of the resistor of FIG. 1 taken substantially along the plane 2-2 thereof;

FIG. 3 is a view in transverse section substantially on the plane 3-3 of FIG. 2;

FIG. 4 is an isometric view showing more clearly the configuration of the core used in a wire-wound resistor embodying the present invention; and

FIG. 5 is an elevational view partly in section of a wirewound resistor embodying a modified form of the invention.

Refer first to FIGS. 2, 3 and 4, which illustrate in detail a wire-wound resistor embodying a preferred form of the invention. The resistor includes a core 10 of any suitable insulating material, such for example, as steatite which is fired after it is shaped to the desired form. The core 10 has a central cylindrical bobbin or body portion 11 with a plurality of formed and spaced transverse integral flanges 12, 13, 14 and 22, 22 which divides the central portion along the core into a plurality of annular cavities for receiving individual multi-layer winding sections 15, 16, 17 and 18 of insulated resistance wire. The resistance wire is continuous, and during winding is passed from one of the winding sections to the next through slots 1'9, 20 and 21 formed in the respective flanges 12, 13 and 14. The insulation of the resistance wire may be any of several conventional ones available for use at 200 C. operation as, for example, a silicone-type of insulation such as a silicone varnish to which one or more suitable fillers have been added. It is found in practice that the ceramic core 10 and the resistance wire both have temperature coeflicients of expansion which are very close to each other over the entire temperature range to which the resistor may be subjected in operation. Strains placed in the resistance wire during winding may change its metallurgical properties, in which event it is usually preferable to subject the wound resistor to several cycles of annealing temperature to stabilize its electrical characteristics.

The central annular cavities for the winding sections 1518 are separated from each end of the core by a pair of annular flanges 23 and 24 spaced from each other and from the end flanges 22, 22. The flanges 23 and 24 adjacent to each end of the core thus define therebetween an endmost terminal cavity 25, while the flanges 22 and 23 at each end of the core define therebetween an inner terminal cavity 26. The flanges 22 are provided with slots 27, corresponding generally to the slots 19, 20 and 21, through which an individual one of the ends of the resistance wire may pass to be electrically connected to an individual one of the lead wires in the annular cavities 26.

The resistor includes axially extending lead wires 28 and 29. In order that these lead wires may be hermetically sealed to the core, each lead wire is inserted axially into the core through an individual hole 30 formed to extend from the end axis of the core into the adjacent annular cavity 26. An hermetic seal is provided between each lead Wire and the core by metallizing in conventional manner the ends of the core about the apertures 30 to provide an adherent metallized bonded area 31 (indicated for purposes of illustration to have exaggerated thickness) and each lead wire is then soldered as shown at 32 to the associated metallized area 31 so as to effect an hermetic seal at this point. Each lead wire will thus be hermetically sealed to the core 10 at its entrance to the hole 30 through which it passes into the core.

Each lead wire is extended into the core for a suflicient distance so that its inner end may be extended out- 3 wardly from the cavity 26 a sufi'icient distance to enable a weld to be made between the end 33 of the lead Wire and an end 34 of the resistance Wire. This weld is conveniently formed with the ends of the wires extending well outside the annular cavity 26, and the wire ends are then bent or folded back into this cavity. The bend thus placed in the end of the lead wire aids in mechanically anchoring it to the core 10.

Following completion of the welding and positioning of the ends of the resistance Wire and lead wires as just de scribed, the entire resistor is wrapped in thin sheets of plastic material having water-resistant characteristics, such as silicone rubber, and the resistor is then placed in a hot mold press to be formed into the desired final cylindrical form illustrated in FIG. 1 and to cure the encasing plastic material by heat and pressure. The pressure used is sufficient to force the plastic material to till all the cavities and the interstices of the structure. The curing time of approximately minutes at approximately 300 F. and the mold pressure leave the end cavities 25 and 26 filled with plastic material under compression so that the material acts as a gasket to seal these cavities against the entrance of moisture. After this initial curing, the resistor is removed from the mold, the flash is cut off, and the plastic material is then further cured for several hours at approximately 125 C. The resistor is thus encased in 'a water-resistant plastic jacket 35 which is left under compression by the molding process and thus provides plural concentric pressure seals at the end cavities 25 and 26, a lighter though effective pressure seal about the lead Wires 28 and 29 adjacent the ends of the resistor, and a light but efiective pressure seal over the end surfaces of the core structure between the lead wires and the end cavities 25, 25. The core configuration employed also substantially lengthens and renders more tortuous the path over which moisture must pass to reach the resistor winding sections 18 should the end pressure seals ever fail for any reason.

From the foregoing it will be seen that the plastic jacket 35, which is per se impervious to the through penetration or transmission of moisture, leaves only such possible spaces for ingress of moisture as exist between this plastic material and the leadwires. Any moisture passing in along the lead wires 28 or 29 and through the apertures 36 of the jacket, which apertures are quite small and are compressed against an associated lead wire, will be prevented from passing further along each lead wire by the hermetic seal formed by the solder 32 and the metallized area 31 at each end of the core. Moisture can, therefore, pass only around the flange 24 and (assuming a failure of the pressure seal in the cavity must then. pass down into the bottom of the annular cavity 25, then out and around the flange 23, then (assuming an additional failure'of the pressure seal in the cavity 26) down and into the annular cavity 26 and thence around the ating temperature changes which characterize the operating conditions under which the resistor of the present invention may often be called upon to operate.

The use of silicone rubber as the plastic material forencasing wirc-wound resistors embodying the present invention has the advantage that this material is capable of withstanding the relatively high operating temperatures and humidity conditions previously described without impairrnent to its resiliency and moisture-resistant characrteristics.

A modified and somewhat simplified form of the invention is illustrated in FIG. 5. In this figure, the core 37 It has been is of insulating material such as steatite and by way of example is shown as provided with a series of similar annular integrally formed flanges 38, 39 and 4t defining in this instance but ltV/O annular cavities 41 and 42 in which resistance wire may be wound as indicated at 43. At each end of the core there is provided an annular integrally corresponding to the cavities 25 used in the earlier form. I

In the present construction, the cavities 45 or 46 correspond generally in function to the cavities 26 of the form of the invention first described. The manner in which the lead Wlifis. are introduced into and are hermeticaily sealed to the core in this form of the invention is substtantially the same as that previously described, so that respectively corresponding parts are given the same refer-' ence numerals. Again, the resistor as a whole is encased in an imperviou integral jacket 47 of a plastic materiai, such as silicone rubber, the jacket 47 being shown in outline'only in dot and dash lines but corresponding in configuration and function to the jacket 35 in the form of the invention first described. 7

In FIG. 5, the lead wires 28 and 29 are shown by way of example as wrapped a complete turn around the core 37 within the cavities 45 and 46. Thus these wires when originally inserted extend far outside of these annular cavities for convenience in gaining more ready access to the ends of the wire for the purpose of welding these ends to the ends of the resistance wire, one of which is shown at 43 in KG. 5. The welded ends when coiled into the annular cavities 45 and 46 following the Welding operation again aid in mechanically anchoring the lead in wires 2 8 and 29 to the core 37 and are then completely enclosed within and by the plastic material of the jacket 47.

While there has beenshown and described but two forms or embodiments of the present invention, it is contemplated that other modifications will occur to those skilied in the art fromthe foregoing disclosure. I do not wish to be limited,; therefore, except by the scope of the appended claims, which are to be construed validly as broadly as the state of the art permits.-

1 claim:

1. A precision wire-wound resistor comprising a winding support core having a central body portion terminated by end grooves each circumscribing said body portion, a resistive wire wound on said body portion with the ends of said wire extending into individual ones of said end grooves, lead wires hermetically sealed through said core and electrically connected in individual ones of said end grooves to individual ends of said resistive wire, each of said lead wires having a bend at the point of its entrance into an end groove mechanically to anchor said each lead wire to said core, and a closed jacket of moisture-resistant resilient plastic material enclosing said core and winding and cooperating with said end grooves to provide moisture-resistant compression seals at each end of said winding.

2. A wire wound resistor in accordance with claim 1,

in which said moisture-resistant plastic material is a silicone-rubber compostion. I

3. A'wire wound resistor capable of operation at elevated temperatures, comprising a bobbin-like core of insulating material having at least one central annular cavity bounded by terminal pairs of axially spaced annular flanges carried by and extend-ing outwardly from said core, each said pair of annular flanges providing an annular terminal cavity therebetween, an insulating resistance wire wound in each said central cavity and having its ends extending'into individual ones of said terminal cavities, lead Wires extending through individual ends of said coreand being electrically connected to individual ends of said resistance wire, metallic means hermetically sealing each said lead wire to said core, and an integral body of moisture-resistant plastic material which retains its moisture-resistant property at the elevated temperature at which the resistor is to be used and which completely envelops said resistor except for outwardly extending ends of said lead wires, said plastic material extending into and substantially completely filling each said terminal cavity.

4. A wire wound resistor cap-able of operation at elevated temperatures: comprising a bobbin-like core of insulating material having at least one central annular cavity bounded by terminal pairs of axially spaced annular flanges carried by and extending outwardly from said core, each said pair of annular flanges providing an annular terminal cavity therebetween; an insulated resistance Wire wound in each said central cavity and having its ends extending into indivdual ones of said terminal cavities; lead wires extending through individual ends of said core and being electrically connected in said terminal cavities to individual ends of said resistance Wire; metallic means hermetically sealing each said lead wire to said core, said metallic sealing means comprising a metallized area bonded to each end or" said core and through which one of said lead wires extends into said core, and solder hermetically bonding each of said lead wires to the metallized spot through which it passes; and an integral body of moisture-resistant plastic material which retains its moisture resistant property at the elevated temperature at which the resistor is to be used and which completely envelops said resistor except for out- Wardly extending ends of said lead wires, said plastic material extending into and substantially completely filling each said terminal cavity.

5. A wire wound resistor capable of operation at elevated temperatures, comprising a bobbin-like core of insulating material having at least one central annular cavity bounded by terminal pairs of axially spaced annular flanges carried by and extending outwardly from said core, each said pair of annular flanges providing an annular terminal cavity therebetWe-en, an insulated resistance-wire wound in each said central cavity and having its ends extending into individual ones of said terminal cavities, lead wires extending through individual ends of said core into individual ones of said terminal cavities and being there welded to individual ends of said resistance wire, metallic means hermetically sealing each said lead wire to said core, and an integral body of moisture-resistant plastic material which retains its moisture-resistant property at the elevated temperature at which the resistor is to be used and which completely envelops said resistor except for outwardly extending ends of said lead wires, said plastic material extending into and substantially completely filling each said terminal cavity.

6. A wire wound resistor capable of operation at elevated temperatures, comprising a bobbin-like core of insulating material having a plurality of central annular cavities, said cavities being separated from each end of said core by three axially-spaced annular flanges integral with and extending outwardly from said core, said annular flanges at each end of said core providing an endmost terminal cavity and an inner terminal cavity, resistance wire Wound in each of said central cavities and having its ends extending respectively into individual ones of said inner terminal cavities adjacent to each end of said core, lead wires extending through individual ends of said core and electrically connected to individual ends of said resistance wire in individual ones of said inner terminal cavities, metallic means hermetically sea-ling each said lead wire to said core, and an integral body of moisture-resistant plastic material which retains its moistureresistant properties at the elevated temperature at which the resistor is to be used and which completely envelops said resistor except for the outwardly extending ends of said lead wires, said plastic material extending into and substantially completely filling each said endmost terminal cavity and each said inner terminal cavity.

7. A Wire Wound resistor capable of operation at elevated temperatures, comprising a bobbin-lilie core of insulating material having at least one central annular cavity separated from the ends of said core by three axially-spaced annular flanges integral with and extending outwardly from said core, said annular flanges at each end of said core providing an endrnost terminal cavity and an inner terminal cavity, an insulated resistance wire wound in each said central cavity and having its ends extending respectively into individual ones of said inner terminal cavities, a lead wire extending axially through each end of said core into an individual one of said inner terminal cavities each said lead wire being welded to an end of said resistance wire in an individual one of said inner terminal cavities, an adherent metallized area formed on each end of asid core surrounding the point of entrance .thereinto or" one of said lead Wires, a solder joint bonding each said lead wire to the metallized area through which it passes so as hermetically to seal each lead wire to said core, and an integral body of silicone rubber completely and closely enveloping said resister except for the outwardly extending ends of said lead wires, said silicone rubber extending into and substantially filling under compression each or" said terminal cavities.

8. A wire wound resistor capable of operation at elevated temperature, comprising a bobbin-like core of insulating material having at least one central annular cavity bounded axially thereof by terminal pairs of axially-spaced annular flanges carried by and extending outwardly from said core, each said pair of annular flanges providing an annular terminal cavity therebetween, an insulated resistance wire wound in each said central cavity and having its ends extending into individual ones of said terminal cavities, lead wires extending substantially axially through individual ends of said core into said terminal cavities and being there electrically connected to individual ends of said resistance wire, an adherent metallized area formed on each end of said core substantially centrally thereof and through which an individual one of said lead wires pass, a soldered joint between each said lead wire and its associated metallized area to provide an hermetic seal between each of said lead wires and the end of said core through which each said lead wire passes, and an integral body of silicone rubber material completely and closely enveloping said resistor except for outwardly extending ends of said lead wires, said silicone rubber extending into and substantially completely filling each said terminal cavity.

References Cited in the file of this patent UNITED STATES PATENTS 2,286,161 Rights et a1. June 9, 1942 2,460,795 Warrick Feb. 1, 1949 2,547,405 Mitchell et al. Apr. 3, 1951 FOREIGN PATENTS 584,549 Great Britain Jan. 17, 1947 

